1. Field of the Invention
The present invention relates to an image forming apparatus, a control method of an image forming apparatus, and a computer program.
2. Description of the Related Art
In trapping raster data, conventionally, a method described in U.S. Pat. No. 6,549,303 can be used to trap a pixel of interest using peripheral pixels (reference area) around it.
In this method, for example, the reference area of an original image is segmented into an area (area A) containing a pixel of interest and another area (area B), and the representative pixels of the two areas are compared, thereby generating a trapping color, as shown in FIG. 9. A trapping color indicates color data to be used for trapping for an unprinted portion generated by misregistration.
For example, if a light color similar to white is present between objects, as indicated by 1001 in FIG. 10, and the objects are apart from each other inherently, trapping should not be executed for a pixel of interest belonging to that portion. In the above-described method, however, trapping is executed by comparing the representative point of the area A with that of an area C. Hence, trapping may be executed for an undesirable part (area which includes a light color similar to white and should not be subjected to trapping).
Additionally, if a large trap width is set, the trap width becomes very large relative to the point width of a character, and the appearance becomes poor. For example, assume an image shown in FIG. 11A. When a character “W” 1101 is placed on a light gray background 1102 and printed using a large trap width, the appearance of the print result is poor, as shown in FIG. 11B. FIG. 11C shows a preferable trapping result.
Assume that a reference area includes a plurality of trapping color candidates. FIG. 12A shows a reference area including three color areas: magenta (M) area 1201, cyan (C) area 1202, and yellow (Y) area 1203. If the plurality of trapping colors are simply combined, the density of pixels rises, and a color such as brown or black that is widely separate from the pixel color of the original document is obtained. For example, when the trapping color M or Y is added to the C area 1202, the color changes to brown or black, like an area 1204 shown in FIG. 12B.
In the above-described technique, how to decide the representative pixel of each of the plurality of areas generated by segmenting the reference area is not examined. The criterion to be used for trapping when the reference area is segmented into a plurality of areas is still a matter for examination. Furthermore, in general trapping, there is a fear of degradation in image quality of a certain kind of object such as a gradation object.
It is difficult to execute a trapping process based on effective reference area setting without degrading the image quality of a printed product.